Recent improvement in the cost on performance of wide band gap semiconductor devices, and GaN HEMT in particular, are making such devices increasingly attractive in high power applications, due to their fast switching and relatively low losses. One example of such high power applications is switch mode power supplies (SMPS).
However the technology with the lowest Rdson and the easiest to manufacture is a normally on HEMT with Schottky gate. In many applications however a normally off switch is needed. This can be made by adding a normal Si-FET between source and gate, in a cascode configuration, such as is known for instance from United States patent application publication number US 2010/0117095. Such an arrangement is shown in FIG. 1 which depicts a cascade configuration 10 comprising a HEMT 12 with its gate connected to a ground 16. The source of HEMT 12 is also connected to ground, by means of a FET 14. That is to say, the FET 14 is connected by its two main terminals in series between the source of the HEMT and ground 16, and in particular the drain of the FET 14 is connected to the source of the HEMT and the source of the FET 14 is connected to the ground. The cascade arrangement is controlled by means of the control terminal 18, that is to say the gate, of the FET.
Precautions have to be made for preventing large currents through the gate of the HEMT, as this gate is vulnerable. It is known to provide gate protection measures for HEMTs, for instance as disclosed in United States patent application publication number US 2007/0228416. Another technique, disclosed in the U.S. Pat. No. 6,614,281 involves disconnecting a cascade circuit with voltage controlled semiconductor switches, in such a way that the gate voltage of a low-blocking-capability semiconductor switches controlled in such a way that its drain voltage is held constant in the active region of the low-blocking-capability semiconductor switch. However, measures that influence the gate have disadvantages, not only due the additional component count, but also have negative impact on the performance, such as reduced switching speed, and the like. It would be beneficial to reduce or eliminate these drawbacks.